Musk xylene is a synthetic musk fragrance which mimics natural musk. It has been used as a perfume fixative in a wide variety of consumer products, and is still used in some cosmetics and fragrances.
Musk xylene was once the most widely used of the "nitro-musks", but its use has declined sharply since the mid-1980s due to safety and environmental concerns. Its explosive and carcinogenic hazards are recognized to be borderline, and musk xylene is a useful example of the lowest level of such risks which need to be taken into account. However, it is a very persistent and very bioaccumulative pollutant in the aquatic environment (vPvB substance), and is the first substance to be proposed as a "substance of very high concern" (SVHC) for these reasons alone under the European Union REACH Regulation. Since no company has applied for authorisation, it is banned in the EU.
Synonyms Musk xylene is also known in english as musk xylol, 1-tert-butyl-3,5-dimethyl-2,4,6-trinitrobenzene, tert-butyl-trinitro-xylene, 5-tert-butyl-2,4,6-trinitroxylene, 1-(1,1-dimethylethyl)-3,5-dimethyl-2,4,6-trinitrobenzene, 2,4,6-trinitro-1,3-dimethyl-5-tert-butylbenzene, 2,4,6-trinitro-3,5-dimethyl-tert-butylbenzene, 5-tert-butyl-2,4,6-trinitro-m-xylene, 5-tert-butyl-2,4,6-trinitro-meta-xylene and xylene musk.
Production and use Musk xylene is produced from meta-xylene (1,3-dimethylbenzene), by a Friedel–Crafts alkylation with tert-butyl chloride and aluminium chloride followed by nitration with fuming nitric acid or with a 70:30 mixture of nitric acid and sulfuric acid. The crude product is recrystallized from 95% ethanol.Bedoukian (1986).
+ Use of musk xylene in various domestic products Product Mass fraction (%) Skin cream 0.0075 Deodorant 0.0075 Shampoo 0.01 Household detergents 0.02 Aftershave 0.03 Toilet soap 0.04 Air freshener 0.07 Cologne/eau de toilette 0.075 Fine fragrance 0.05–0.1 Sources: International Agency for Research on Cancer (1996); European Union Risk Assessment Report (2005). NOTE: Use of musk xylene varies widely between different countries and manufacturers; these figures should be regarded as indicative maxima for the period 1990–present. Musk xylene has been used in a wide variety of consumer products since the early 1900s, usually in very small quantities. World production of nitro musks in 1987 was about 2500 tonnes, but had fallen to about 1000 tonnes by the early 1990s: musk xylene made up roughly two-thirds of the production of nitro musks during this period. Production was concentrated in Western Europe, with the United Kingdom alone accounting for 28% of world production of nitro musks.International Agency for Research on Cancer (1996).Ippen (1994).
Use of musk xylene continued to decline through the 1990s, as fragrance manufacturers voluntarily switched to alternative fragrance compounds.OSPAR Commission (2004). For example, musk xylene has not been used in Japanese products (on a voluntary basis) since 1982, and the Association of the German Toiletries and Detergents Industry (IKW) recommended the replacement of musk xylene by another compound in 1993.Section 2, European Union Risk Assessment Report (2005), pp. 9–10. Production of musk xylene in the European Union came to a halt and, by 2000 (the last year for which full data are available), imports to Europe were only 67 tonnes, with China as the most important source. The estimated 2008 usage of musk xylene in the European Union was 25 tonnes.RIVM–DHI–RPA (2008).
Musk xylene permitted until 2011 for use in cosmetics products (except oral care products) in the European Union under the Cosmetics Directive. The permitted quantities are: up to 1% in fine fragrances; up to 0.4% in eau de toilette; up to 0.03% in other products.ATP (2004) to the Cosmetics Directive. European Union suppliers must inform their customers on request if a product contains more than 0.1% by weight of musk xylene.Article 31.3, REACH Regulation, at p. 108. In 2011, it is listed as a product to "be banned within the next three to five years unless an authorisation has been granted to individual companies for their use".
Safety Musk xylene is an analogue of the explosive trinitrotoluene (TNT), so it is unsurprising that its safety characteristics have been studied in some detail. Indeed, the nitro musks were first discovered in an attempt to produce new high explosives. It has also been used – albeit in very small amounts – in mass-market consumer products for the last hundred years. The discovery of musk xylene residues in the environment prompted new concerns about its possible long-term toxicity, and led to the sharp decline in its use from the mid to late 1980s. The European Chemicals Agency has listed musk xylene as a "substance of very high concern" (SVHC) under the REACH Regulation, judging it to be "very persistent and very bioaccumulative" (vPvB) but not meeting the criteria for human or environmental toxicity to be of concern.European Chemicals Agency (2008).
Explosive properties Musk xylene is used as an example case in the United Nations Manual of Test Methods and Criteria as a substance which shows some explosive properties but which does not have to be transported as Class 1 dangerous goods under the Model Regulations.Section 10.5, Part I, UN Manual of Tests and Criteria, pp. 23–28. It is transported as small flakes in plastic bags (maximum 50 kg net mass), which are themselves within cardboard drums to avoid tearing.Section 4.1.1.2, European Union Risk Assessment Report (2005), p. 42.Packing instruction P409, Section 4.1.4, Part 4, UN Model Regulations, at p. 59. This does not count as "confinement" in the meaning of explosives tests: indeed, the special packing is intended to prevent over-confinement during transport.Special provision 133, Section 3.3.1, Part 3, UN Model Regulations, at p. 291.
It will explode when detonated under confinement (UN gap testSection 11.4, Part I, UN Manual of Tests and Criteria, pp. 32–34.) or when heated under confinement (Koenen testSection 11.5, Part I, UN Manual of Tests and Criteria, pp. 35–40.), but does not explode under the BAM fallhammer testSection 13.4.2, Part I, UN Manual of Tests and Criteria, pp. 76–83. (limiting impact energy 25 J) or the BAM friction testSection 13.5.1, Part I, UN Manual of Tests and Criteria, pp. 105–8. (limiting load >360 N). There is no ignition, explosion, self-heating or visible decomposition when musk xylene is heated (without confinement) to 75 °C for 48 hours.Section 13.6, Part I, UN Manual of Tests and Criteria, pp. 117–19.
Nevertheless, musk xylene is classified in the European Union as an explosive under the Dangerous Substances DirectiveATP (2004) to the Dangerous Substances Directive, at p. 121 (index no. 609-068-00-1). and as a category 1.1 explosive under the CLP Regulation.CLP Regulation, at p. 615 (index no. 609-068-00-1). The European Union classification reflects the fact that hazardous heating under confinement cannot be excluded in the industrial use of musk xylene, as opposed to its transport, and so it is necessary to warn potential users of the risk.Position document prepared for the European Chemicals Bureau (2002).
Carcinogenicity Musk xylene also demonstrates some of the problems of classifying substances as carcinogens. It has been placed into Group 3 ("not classifiable as to their carcinogenicity to humans") by the International Agency for Research on Cancer (IARC), and is classified in the European Union as category 3 carcinogen ("cause concern for man owing to possible carcinogenic effects but in respect of which the available information is not adequate for making a satisfactory assessment") under the Dangerous Substances Directive and a category 2 carcinogen ("suspected human carcinogen") under the CLP Regulation.
These classifications are based mainly on a single study of oral exposure to musk xylene in B6C3F1-strain mice.Maekawa et al. (1990). The mice showed a highly significant increase in liver adenomas and carcinomas at median dietary intakes of 170 mg/kg body weight (males) and 192 mg/kg body weight (females), as well as significant increases in adenomas in the Harderian gland (male mice only) and in the liver at median dietary intakes of 91 mg/kg body weight (males) and 101 mg/kg body weight (females).
The European Union Risk Assessment Report makes a number of observations about this study:Section 4.1.2.7.3, European Union Risk Assessment Report (2005), pp. 83–85. it was conducted on a single species; no studies are available on, for example, rats; B6C3F1-strain mice are known to be particularly prone to liver cancers; the doses were high, and toxic effects (especially on the liver) were observed in the test animals; the mechanism of tumour development is unclear.
Musk xylene is not genotoxic.Section 4.1.2.6, European Union Risk Assessment Report (2005), pp. 74–77. It has significant effects on liver function which are similar to those shown by phenobarbital, for example, induction of CYP2B6 and other cytochrome P450 enzymes.Section 4.1.2.7.1, European Union Risk Assessment Report (2005), pp. 77–83. The human carcinogenicity of phenobarbital has been the subject of debate,Williams & Whysner (1996).International Agency for Research on Cancer (2001). but it is currently classified in group 2B by the IARC and this appears to have been an important consideration in the classification of musk xylene as a category 3 carcinogen under the Dangerous Substances Directive.Meeting of the Commission Working Group on the Classification and Labelling of Dangerous Substances, 25 November 2002. Nevertheless, the European Union Risk Assessment Report admits that musk xylene is a "borderline case".
A further complication is the metabolism of musk xylene. One route of metabolism is through reduction of one or more nitro groups by the intestinal microflora (gut bacteria) to produce aromatic amines such as p-NH2-musk xylene. This metabolite has a different liver toxicity: in particular, it inhibits the CYP1B enzymes by covalent binding.
Induction of cytochrome P450 enzymes, the most likely cause of rodent carcinogenicity, is a threshold phenomenon, with a no observed effect level (NOEL) of 10 mg/kg/day in mice and a lowest observed effect level (LOEL) of 10 mg/kg/day in rats. The lowest oral dose which caused cancer (LOAEL) in B6C3F1-mice was 70 mg/kg/day. These are 1–3 orders of magnitude higher than human exposure, which is principally dermal rather than oral.Sections 4.1.1.5 & 4.1.3.5, European Union Risk Assessment Report, pp. 57 and 109–110.
Environmental concerns The first concerns about musk xylene arose in the early 1980s, with the detection of musk xylene residues in fish from the Tama River near Tokyo,Yamagishi et al. (1981). and subsequently in the river water itself, especially at the outlets of sewage treatment plants. This led to a voluntary moratorium on the use of musk xylene in Japan from 1982. Similar residues were subsequently found in European waters such as the Elbe, Stör and Ruhr rivers in Germany, the German Bight area of the North Sea and sewage treatment plant outlets in Sweden.Section 3.1.2.4, European Union Risk Assessment Report (2005), pp. 21–24. Typical concentrations were >0.001 μg/L in sea water, 0.001–0.01 μg/L in river water and 0.01–0.1 μg/L (sometimes higher) in the effluent from sewage treatment plants.